> What is Loeb's reputation among astrophysicists?
Not an astrophysicist but several friends are. The consensus seems to be that he was a good scientist at one point but has turned into a sensationalist attention monger.
> He's written "over 800 scientific papers". Is that... a good thing in his field?
He's been an author on over 800 scientific papers: that includes his work, work he's been a significant collaborator on, and student work he's supervised. 800 is a lot for someone his age but not unheard of. People care much more about paper quality (or less charitably, citation count) than quantity.